Traveling-wave tubes



Oct. 17., 1961 c. c. cu'rLER TRAVELING-WAVE TUBES /M/EA/ro/Q C. C. CUT/ER Oct. 17, 1961 c. c. cuTLER TRAVELING-WAVE TUBES SPACE CHARGE NO SPACECHARGE /0 GA//v ro oUrPur BCA/v DEC/@ELS 7'0 OUTPUT /NVE/V? C. C. CUTLER22.149. 4,

ATTORNEY Oct. 17, 1961 c. c. cuTLER TRAvEuNG-WAVE TUBES 5 Sheets-Sheet 3Filed June' l5, 1950 f f r/ jo N L// i N f -6C /0 /w m W1 m c u w V 0 G/-um U t 0 B c m 8 m E G. M G/ 0 D 0 -4W |4M n N c R Q w w w n W 0 m n Wm W 0 .Num El d .5u l

C 8 m. F 5 0. ,4 C Q 0 M w M m dkuwm Oct. 17, 1961 c. c. CUTLER3,005,126

TRAVELING-WAVE TUBES NORMAL/25p D/rA/vcf To UrPUr c/v' Arrop/vgy Oct.17, 1961 c. c. cuTLER TRAVELING-WAVE TUBES 5 Sheets-Shree?l 5 Filed Junel5, 1950 ATTOP/VE 3,605,126 LTRAVEUNGQWAVE TUBES Cassius C.Cutler,--Gillette`, Nr.3., assigner to Bell Telea phone.V Laboratories,`incorporated, New York, NX., `a corporation` of New York Filed .lune l5,1950, Ser.jNo. 168,29?. 20 Claims. (131. S15- 3.5)

This inventionurelates generally to wide band microwave amplifiers andymore particularly to traveling-wave tubes, in which the interactionbetween a stream of electrons and a traveling Aelectromagnetic wave isutilized to secure gain.

One object of the invention is to enable maximum power output .and`efficiency to be `obtained from a traveling-wave tube.

Another. object is to maintain stability and avoid undesirable impedanceeffects in -all portions of a travelingwave tube.

A further obje-ct of the invention is to reduce the size and powerrequirements of the `electron beam focusingI means of thetube.

.Still another object is to .eliminate possible irregularities in themagnetic field produced by the electron beam focusing means.

A still further object is to avoid loss of gain caused by widedifferences of :electron velocity within the electron stream.

In the past, it has generally been necessary `to provide traveling-Wavetube circuits with high frequency attenuation or loss `in `order ttomaintain stability andavoid `had long-line impedance effects. It isextremelydiflicult to secure accurate impedance-matches `between thetraveling: wave cireuitand` the-signal input and output circuits overthe broadfrequenc-y range in whicha traveling-wave tube operates.Components of `the radio frequency signal wave tend, therefore, Ato be,reflected back and forth-.along the traveling-waveV circuit. Such`components traveling in the direction of electron 'flow are amplied andmay produce oscillations, resulting in tube instability.A When suchcomponents are reflected back tothe input endl of the traveling-waveycircuitzout ofrphase withithe incoming signal wave, the signal wave isdegraded, resulting `in what may be termed 4long-'line impedance effectsReflected `components .ofwthe .rad-io frequency signal wave tend to beabsorbed by attenuation, which, as disclosed in application vSerialNo...6l0,15'97, `filed lanuary lll, 1946, by Pierce, now United StatesPatent 2,636,948, issued AprilnZS, 1953, may be `introduced into thetraveling-wave circuit. If the total circuit attenuation iis'conrparable `in magnitude to the over-'all gain of thetube, instabilityand long-:line impedance effects may be effectively reduced. SIn .thepast, circuit attenuation or loss has been eitherlumpedat some pointalong the traveling-Wave circuit or distributed uniformly over most ofthe length of tlietcircuit.- However, undertcertain conditions, bothmethods` present -dinicultieswhich it is desir'- able to ove'rcoine.nbetter results may be obtained by 4following' the procedures which aretobe described.

In accordance with one feature of the present in'vention, the high`frequer'icy attenuation or loss distribution in a traveling-wavetubeiis such that the tube may be operated at Inaiiimum power andeiciency while avoiding instability andI `undesirable impedance effects.`The power referred to is the signal output power, while the eniciencyreferred to is the ratio ofthe signal output power to the direct-currentinputV power. Short sections of substantially lossless lcircuit are leftat both ends' of the tube and the attenuation is distributed Aalong acenter section intermediate .the end sections so that the. attenuationper unit length isgreater near the' input end` than it 'Iliidg StatesPatent O ICC is near the output end. To make up the center section of,distributed loss, a relatively short section of very high attenuationper unit "length may be followed by a relatively long section of onlymoderate attenuation per unit length or the attenuation per unit lengthmay be at a maximum nearthe input end of the section and decreasegradually to substantially 'zero at the output end. In both examples,the total attenuation over the length of the traveling-wave circuit iscomparable in magnitude -to or greater than the net gain of the tube.The first-mentioned embodiment, in which a relatively short section ofhigh attenuation per unit length is followed by a relatively longsection of only moderate attenuation p'er unit length, represents thedistribution affording maximum gain for a given length consistent withmaximum power and efliciency. The second, in which the attenuation perunitlength is at a` near the input end of the section and decreasesgradually to substantially zero at the output end, represents thedistribution affording maximum stability and freedom from longlineimpedance effects consistent with maximum power and eiciency. By way ofexample, the present feature of the invention is found to make possibleetlciencies of the order of ten per cent, as compared with the one-halfof one per cent to ve per cent, varying from tube to tube, which werepreviously available.

In the operation of traveling-wave tubes, it has generally been foundnecessary to supply some kind of a magnetic focusing eld to maintain the`desired electron beam dimensions over relatively long distances. Oftenthis is accomplished by placing the whole beam and associated structurein fthe strong uniform magnetic field produced 4by a solenoid. The usualresult is a fairly bulky structure requiring considerable focusing powerand thus impairing the total power elicienoy of the tube.

In accordance with another feature of the present invention, `permanentmagnets are employed as auxiliaries to' a solenoid in a composite`system and `the bulk and power requirements of the focusing system arethereby considerably reduced. The traveling-wave circuit is within the`i`e1d of the solenoid in its portion lying between the signal input andoutput circuits, and the .un-iform eld is` entended past the input andoutput `circuits by means of the permanent magnets. Soft steeltransverse plates may be employed to straighten out any irregularitieswhich appear iu thefocusing field.

In the past, it has also been found that the gain theoreticallyavailable from a traveling-wave tube is reduced due to a velocityspreadwithin the electron stream. Electronswliich are out of synchronismwith `the traveling wave do not contribute theirfull share to the gainof the tube and the over-all gain is thereby reduced.

In accordance with still another feature of the inventionp the electrongun is shielded from the magnetic focusing eld and the electron streamis subjected to the field abruptly at the point of minimum beamdiameter. The electrons are thereby made to new in parallel helicalpaths and the direct-current beam velocity is constant across the beam.

In addition, beam focusing problems are minimized and much betterelectron transmission than would otherwise be possible is secured.Transmission approaches one hundred percent for very dense beams andvery few electrons are lost to the wave transmission circuit in transit.

Additionalobjects and features of the present invention will appear froma study of the following detailed exposition. In the drawings, which aresubstantially to' scale:

FIG. l illustrates a `traveling-wave tube as it appears vvithoutsignal`input and output circuits, focusingmeans,

and miscellaneous attachments;

FIG. 2 is a transverse cross section of the tube of FIG. l showing therelationship of the wave transmission helix, the helix-support rods, andthe glass envelope of the tube;

FIG. 3 is a longitudinal cross section of the tube of FIG. l, completewith input and output circuits, focusing means, and miscellaneousattachments;

FIG. 4 shows the helix-support rods of the FIG, 1 tube with a schematicrepresentation of one of the loss distributions within the scope of thepresent invention;

FIGS. 5 through 9 are graphs showing significant relationships involvingattenuation distribution;

FIG. shows the magnetic field distribution at the input end of thedescribed tube; and

FIG. ll shows two different devices for eliminating irregularities inthe focusing eld.

Referring particularly to FIG. l, the traveling-wave tube shown isenclosed in an elongated glass envelope 21. Envelope 21 includes anVenlarged portion at its left end to house theV electron gun assembly,but is of uniform diameter over most of the rest of its length, where ithouses the signal wave transmission circuit. Envelope 21 is closed atboth ends, thus enabling the tube to be evacuated.

FIGS. 2 and 3 show the structural details of the tube. The electron gunsupporting assembly is of a type well known in the vacuum tube art andis therefore not shown. While schematic connections are shown to severalof the gun elements, it is to be understood that they are actually tothe tube base. FIG. l shows the general outline of the gun supportingassembly and the tube base to which actual connections are made.

Referring to FIG. 3, an electron-emissive cathode 22 is located withinthe enlarged left end of envelope 21. Cathode 22 may be in the form of aconducting plate with the axis of envelope 21 normal to its broadsurfaces. Cathode 22 is aligned with the hollow interior of thesmall-diameter portion of envelope 21. The righthand face of cathode 22may be concave.

A heating coil 23 is located just to the left of cathode 22 and isshielded by a short tubular conducting member 24. Member 24 is attachedto the left-hand face of cathode '22 and is of substantially the samediameter as cathode 22. One side of heater 23 is conductively connectedto member 24 and to one side of a heater supply battery 25. The otherside of heating coil 23 is conv nected to the other side of battery 25.

' An electron beam forming electrode 26 surrounds and is coaxial withcathode 22. Electrode 26 `is a hollow member, the inner surface of whichforms an extension of the concave face of cathode 22. The ,inner surfaceof electrode 26 is defined, in effect, by a pair of coaxial rightcircular truncated cones, the portion of the surface nearest cathode 22being defined by a cone of smaller altitude than the cone dening theportion of the surface farther to the right.

Tubular member 24 is located largely within the lefthand portion ofelectrode 26 and is surrounded by a non-conducting ceramic cylinder 27.Cylinder 27 ts against the inside surface of the left-hand portion ofelectrode 26 and is spaced slightly apart from electrode 26 at its rightend by an annular non-conducting spacer 28. An annular conducting plate29 surrounds tubular member 24 and tits against the left-hand end ofcylinder 27. Three screws 30 are equally spaced around the outsideportion of plate 29 to hold it in place and extend into the left-handface of electrode 26. The end of heating coil 23 that is connected totubular member 24 is also connected to plate 29, thereby holding cathode22 and beam forming electrode 26 at the same potential.

An anode electrode 31 is located to the right of beam forming electrode26 and comprises a relatively flat circular flange-like portion and atubular nose portion extending into the small-diameter portion ofenvelope 21. The flange-like portion of anode 31 is at the left and isaligned with electrode 26. VA central aperture in anode 31 is alignedwith the axis of envelope 21 to permit the passage of electrons. Aroundthe aperture, the left-hand end of anode 31 extends to the left towardcathode 22, the extension being in the form of a section of a rightcircular truncated cone. Anode 31 is spaced from and electricallyinsulated from beam forming electrode 26 by a ceramic ring 32. Anode 31,ceramic ring 32, and

. beam forming electrode 26 are held together by three screws 33, whichare spaced about the periphery of the respective elements. The screws 33are insulated from electrode 26 but are joined directly to anode 31,thereby enabling the potential of anode 31 to be fixed by theapplication of a potential to one 0r more of the screws 33.V

The structure which has been described forms an electron gun. The gunassembly is held in place within the enlarged left-hand end of envelope21 by the elongated nose of anode 31 and by leads which are connectedto'- one or more of the screws 33. The cathode heating element 23 isheld in place by the leads connecting it to the tube base and cathode 22and tubular member 24 are positioned by the lead connecting thelatterelement to one side of heating coil 23.

To the right of the nose portion of anode 31, an elongated wire helix 34extends through most of the small-diameter portion of envelope 21. Helix34 is of substantially uniform pitch throughout most of its length andmay be wound so that the ratio of its length to the total length of wireforming it is about one to thirteen. At both ends, the pitch of helix 34is gradually increased for impedance matching purposes. Helix 34 ispositioned within and separated from envelope 21 by four ceramicsupporting rods 35 which are spaced about its periphery. FIG. 2 showsthe relative positions of helix 34, rods 35, and envelope 21.

The left-hand ends of the rods 35 are supported by a collar 36 which isin the form of a hollow conducting cylinder. Collar 36 contains slotsspaced about its righthand end to receive the left-hand ends of rods 35and a conducting strip 37 extends to the right between two of therods 35at the top of the tube, The left-hand end of helix 34 is attached to theright-hand end of strip 37. A ceramic ring 38 separates the left-handend of collar 36 from the right-hand end of the nose portion of anode31. e

The right-hand end of rods 35 are'supported by a collar 39 which is thesameV as collar 36 at the other end of the tube. A conducting strip 40is the same as strip 37 and extends from collar 39 to the right-hand endof helix 34. A ceramic ring 41 is located to the right of collar 39 andsupports a collector electrode 42. The left-hand end of collector 42 isin the form of a short tubular conductor and the right-hand end is inthe form of a hollow conducting cone. A lead 43 is attached to theright-hand end of collector 42, contains a short coiled section to serveas a high frequency choke, and extends through the right-hand end ofenvelope 21. A lead 44 is attached to collar 39, passes through ceramicring 41, and also extends through the right-hand end of envelope 21.

The radio frequency signal wave which is to be amplied is applied tohelix 34 through an input wave guide 45. Wave guide 45 is a standardhollow rectangular wave guide and is closed at one end. Envelope 21 eX-tends through apertures in the walls of wave guide 45 with its axisnormal to the broad surfaces of the guide. The right-hand end of collar36 is flush with the inside surface of the left-hand wall of wave guide45 and conducting strip 37 extends about half-way into the guide. Strip37 is midway between the narrow side walls of the guide and issubstantially one-fifth of a signal wavelength from the closed end. e

The amplified signal -is withdrawn'from the other end of helix 34through an' output Awave guide 46. Output guide 46 is substantially thesame as input guide 45 and the inside surface of its right-hand wall isush with theanotarse halffway intothe guide and is locatedtsubstantiallyonefth .of a wavelength from` its closed end.

. Operating potentials are supplied from a `heater battery 25.,` aspreviously discussed, and from a main directcurrent source 47. The mostnegative point` of source 47 is connected to the base prong whichconnects to cathode 22, while an intermediate positive point isconnected to the base prong which connects to anode 31.

The most positive point is connected to lead 44 to determine thepotential of helix 34 and a slightly `less positive point is coupled tolead 43` to determine the voltage of tcollector 42. The illustratedbattery connections of anode 31, `lead 43, and lead 44 are shown by wayof example. Other relative potentials may be used.

When heater 23 is energized by battery 25, the electron gun projects aconverging electron beam to the right through the central aperture ofanode 31 and lengthwise through helix 34. At the right-hand end of thetube, the electrons are collected by collector electrodet42. order toconfine the moving electrons to the relatively narrow path provided forthem, a strong longitudinal magnetic focusing field is provided. To thisend, the portion of envelope 21 between wave` guides 45 and 46 issurrounded by a solenoid 48. A non-magnetic tubular member 49 fitsbetween the exterior of envelope 21 and the inside surface of solenoid48', and atubular magnetic shield 50 fits around the outside surface ofsolenoid 48.` Solenoid '48 is supplied with direct current `by a`battery 51.

A pair of soft steel end plates 52 and 53 arelocated at respective endsof solenoid 48. End plates 52 and 53 have central apertures into whichthe respective ends of Solenoid 48 and shield 50 are fitted. The axis ofenvellope 21 is normal to both end plates 52 and 53. The left-.hand faceof end plate 52 is substantially flush with the left-hand end ofsolenoid 48, while the right-hand face of end plate 53 is substantiallyflush with the righthand end of solenoid 48. A second tubular magneticshield 54 is concentric with and surrounds shield 50. Shields 50 and 54are separated slightly from each other and shield 54 extends between theright and left-hand faces, respectively, of end plates 52 and53.

A soft steel end plate 55 also fits around envelope 21 at its neck, thatis, near the portion of the reduced diameter part of envelope 21 that isnearest the enlarged guncontaining part. A pair of concentric tubularmagnetic shields 56 and 57 extend from the left-hand side of end plate55 and enclose the portion of envelope 21 housing the electron gunstructure.

A pair of permanent bar magnets 58 are bridged between end` plates 52and 55 on either side of input wave guide 45 to extend the magneticfocusing field set up by solenoid 48. Each magnet S8 is parallel withthe axis of envelope 21.' A short non-magnetic collar 59 fits closelyaround envelope 2 and extends to the left from the left wall of inputwave guide 45. To straighten out possible defects in the magnetic field,a circular soft steel plate 60 is fitted` against the outside of theleft-hand wall of input wave guide 45. Plate 60 ts outside of collar 59and `extends perpendicularly to the magnetic field. Another `soft steelplate 61 is fitted against the outside of the right-hand wall of inputwave guide 45 and performs a similar function.

A circular soft steel plate 62, which is substantially identical toplates 60 and 6l, fits against the outside of the left-hand wall ofoutput wave guide 46. Plate 62' has a central aperture to accommodateenvelope 2l andextends perpendicularly to the magnetic focusing field.A- shortnon-magnetic collar 63 surrounds envelope 21 and extends to then'ght from the right-hand wall of output Wave guide 46. A circular softsteel plate 64 fits against the outside of the right-hand wall of outputguide 46 and extends perpendicularly to the magnetic field to straightenout possible irregularities. To the right of plate 64 are located a pairof resonators 65 and 66, which serve as radio` frequency chokes toprevent the signal Wave from being transmitted beyond them. Resonator 65is between resonator 66 and plate 64. The resonant cavities withinresonators 65 and 66 are of theannular re-entrant type, and collarl 63is` part of the struct-ure defining the cavity of resonator 65. A softsteel end plate 67, corresponding to end; plates 52, 53, and 55, -isfitted around the exterior of resonators 65 and 66 and extendsperpendicularly to the axis ofenvelope '21. Aipair of permanent barmagnets 68 are bridged between end plates Sand 67 on either sideofoutput wave guide .-46 to extend the magnetic focusing eld of the tube.Magnets 68 are parallel to the axis ofenvelope121 andare outside theside walls ofoutput wave guide 46.

In the operation of the described traveling-wave tube, an electronstream is projected, as` previously discussed, from cathode 22 to`collector 42. The stream is con fined to its path `by the stronglongitudinal magnetic focusing field setup by Vsolenoid, 48`andpermanent magnets 58 and 68. The incoming signal wave energizescoupling strip 37, which serves as an antenna, and is therebytransmittedtohelix 34. Thewave travels along helix 34 at a velocityapproximating` that of the electron stream and is caused to grow in`amplitude by interaction between its electric field components and theelectron stream. At the right-hand end of helix 34, the amplified waveenerg'izes coupling `strip 40, which in turn excites a wave ofcomparable amplitudezin output wave guide 46. From there the amplifiedwave may be applied to a suitable load circuit.

By way of example, `the following `values are typical of a number oftubes ofthe type described:

In the past, helices of travclingwave tubes hayebcen provided with radiofrequency attenuation or loss to enhance stability and to avoidundesirable impedance effects. Even with the impedance matching schemeshown in FIG. 3*, where the end turns of helix 34 are increased in pitchas the `tips of .the coupling antennas 37 and 40 are approached, it isAextremely difficult to secure exact impedance matches between helix 34and input and output Wave guides 45 and 46, over the entire frequencyrange in which the tube operates. Components of the radio frequencysignal wave tend to be reflected back I and forth along helix 34. Suchcomponents are amplified by interaction with the electron stream whenthey travel in the direction of electron ilow and the tube tends to beunstable. When' such componentsare reflected back to` the input endofhelix 3'4 out of phase'with the incoming wave from input wave guide45, the signal tends `to be degraded and longiine impedance eiectsresult.

As has previously been indicated, reflected components of the radiofrequency signal wave tend to be absorbed by attenuation or loss in thetraveling-wave circuit when such attenuation is introduced along thewave path. Such attenuation or loss may be introduced, by way ofexample, by applying a thin coating of carbon or other conductingmaterial on the helix supporting rods 35. In the past, such attenua-tionhas been either lumped at an intermediate point along the length of thehelix or distributed uniformly over most of its length. lf the totalloss int-roducedis comparable in magnitude to the net gain of the tube,instability and long-line impedance effects may be effectively reduced.However, as has also been` mentioned previously, `lumped and `uniformlydistributed atteuuations present diiculties under certain conditionswhich it is desirable to overcome. If lumped attenuation is positionedalong the helix to give maximum power capacity, the tube tends tooscillate and undesirable impedance elfects tend to appear. If theattenuation is uniformly distributed, there is danger of instability ifthe attenuation per unit length is very great and is applied to allowmaximum eicie'ncy, and the tube tends to be inconveniently long if asmall value of attenuation per unit length is used.

-In accordance with a feature of the present invention, the attenuationis distributed along the traveling-wave circuit so that the tube may beoperated at maximum power and efficiency. Instability and long-lineimpedance effects are largely avoided and the tube need not beinconveniently long. The attenuation or loss material may, by way ofexample, be deposited on the helix supporting rods 35 to introduce theproper loss into the traveling- Wave circuit. Sprayed colloidal graphiteand pyrolytically deposited carbon are two examples of such de positedloss material.

FIGS. A and 5B are curves showing preferred attenuation distributions inaccordance Vith a feature of the present invention. In each figure,curve A represents decibels attenuation per unit length plotted againstdistance measured along the tube from the input circuit and curve Brepresents signal level in decibels plotted against the same quantity.The scales in each figure are linear and begin with zero.' In general,in accordance with this feature of the invention, short sections ofsubstantially lossless circuit are left at both ends of the tube and theattenuation is distributed along a center section intermediate the endsections so that the attenuation per unit length is at least severaltimes greater near the input or upstream end of the center section thanit is near the output or downstream end.

Y To make up the center ysection of distributed loss, in the embodimentof this feature of the invention diagramed in FIG. 5A a relatively shortsection of very high attenuation per unit length is followed by arelatively long section of only moderate attenuation per unit length.The high attenuation per unit length is at least several times themoderate attenuation per unit length and the distribution shownrepresents the design for maximum gain for a given length consistentwith maximum power output and elciency. By way of example the highattenuation per unit length may be thirty decibels per inch while themoderate attenuation per unit length may be three decibels per inch. Inthe embodiment diagramed in FIG. 5B, the attenuation per unit length ismaximum near the input end of the lossy section and decreases graduallyto substantially zero at the output end. This distribution representsthe design for maximum stability and freedom from undesirable impedanceeffects, consistent with maximum power output and eiciency. The maximumattenuation per unit length may be of the order of ten decibels perinch. In both embodiments, the total attenuation over the length of thetraveling-wave circuit is comparable in magnitude to or greater than thenet gain of the tube.

FIG. 4 illustartes the helix supporting rods 35 of the described tubewith loss material deposited according to the distribution of lFIG. 5A.As previously noted, colloidal graphite may be sprayed onto the rods 35to give the desired distribution, or carbon may be depositedpyrolytically. Numerous other methods of depositing the loss may also bedevised.

IFIG. 6 shows several curves which may be used to determine the lengthof the substantially lossless output region of the FIG. 5A attenuationdistribution. The scales are linear and begin with zero. The solidcurves represent the ratio of attenuation per unit length to gain perunit length in a lossless region, plotted against gain to the output endof the circuit in decibels. The upper curve covers the case where thespace charge of the lelectron stream is appreciable and the lower curvecovers the case Where it is neglected. Whichever condition may obtain,the solid curve represents the maximum permissible value of the ratiofor particular distances upstream from the output circuit from thestandpoint of power output. If the indicated value of the ratio isexceeded, power output tends to be reduced. The horizontal dashed lineof FIG. 6 indicates a value of the ratio equal to unity. Where loss isapplied uniformly the value of. the ratio should be greater than unityfor stability to be achieved.

With regard to their application to the distribution of FIG. 5A, thesolid curves of FIG. 6 indicate how much attenuation may be applied at agiven point with` out limiting the power output and eiciency. Thefarther back, that is, upstream, from the output the loss is applied,the greater is the loss that may be applied. The dashed line representsa limiting value below which the loss should not go when a uniformdistribution is used, the uniform distribution being the section ofmoderate loss shown in FIG. 5A.

I'he curves of FIG. 6 may also be applied to the distribution of FIG.5B, in which the maximum attenuationV consistent with high power outpu-tis used, thereby holding long-line effects and instability problems to aminimum. As far back toward the input as the upstream end of the lossysection, curve A of FIG. 5B corresponds to the applicable solid curve ofFIG. 6, the dashed curve being disregarded. The zero point of the solidcurve indicates the `downstream end of the lossy section and the valueof the attenuation per unit length at all `upstream points is determinedby successive points on the curve.

FIGS. 5 and 6 are included in the description ofthe attenuationdistribution feature of the present invention to show the distributionsfrom one point of view. The following discussion includes thepresentation of curves showing the distributions from another point ofview.

In tube designs incorporating the present invention, it is desirable tohave suicient circuit attenuation to guarantee stability underpractically any foreseeable input' and output conditions and to have anadditional attenuation margin in order to minimize long-line impedanceeffects. In order not to limit the power output unnec-V essarily, theoutput section of the traveling-wave circuit should be nearly lossless.Also, in order to start inter-' action at the input end of the tubesatisfactorily, an input section of minimum loss is desirable.Therefore, in accordance with a feature of the present invention, thecircuit comprises two lossless sections with a section of distributedloss between them in which the loss's distributed in a specified manner.

When a relatively long circuit section of substantially uniform loss isfollowed by a substantially lossless output section, in order to avoidoverloading effects in the output section not only should overloadingdetectableV in the intermediate lossy section be avoided, but the tubeshould be operated at a substantial margin below the level at which itappears. By test, it appears that if the, signal level at the downstreamend of the lossy region is more than six decibels below the overloadlevel for that loss, the overload level being that level where outputceases to rise as the input level is increased, the output power willnot be seriously limited.

At complete overload, the gain in the output section may be compressedby three decibels or more. Consequently, for the tube to tbe able todeliver the maximum power the minimum low level gain following auniformV distribution of loss should be these three decibels, plus sixdecibels for overload margin within the lossy region, plus thedifference between the overload levels at the output end of the lossyregion and the output end of the tube. It can thus be seen that thereshould be a rather large amount of gain following even a moderate amountof attenuation.

From the foregoing conclusions, an attenuation, dis` tributiondesignrfor maximum power may be obtained.

, *9 Following any v section of I.uniform attenuatng, :there should be aminimum amount oflowlevel gainthat amount of gain depending upon theamount of attenuatfzgpreceding it. this limitation is insmeren by curve.A inFIG. 7, where the maximumpermissible. attenuationnper unit lengthat `any `pointis` plotted 1as a function of; a parameterwhchuormalizesidistauce measured backin the` upstream .direction from;theoutput. Curve A` corresponds tothe solidcunvesof 6 andrepresents amaximum value beyond which-,the attenuation permuit length should hongo.The curve, it will be noted, tappliesstrictlyonly when ther loss-isuniform from the `point in .question back toward the input.

Ilieordinate of curve A `of lFIG. 7 is-the Aloss iactor L/C, where Lis`the .attenuation in decibels per` circuit wavelength and C is :aparameterfor gain perunit length which depends. upon thecircuitandtelectron beam impedances. C is given by the relation where`Eis the `electric iield actingon the beam` `at any point in. thedirection of wave propagation, is a, phase constant, P `is `thetransmitted power at any point along the circuit in watts, I is thedirect beam current in amperes, .and V0 is the direct beam voltage. is`given by the, relation where o isthefsignal frequency in radians vpersecond, v is the direct-current beam velocity in metersper `second, .andkg is the circuit wavelengthin meters.

VThe abscissa of allthe curves of PIG. 7, including curve .AA, is CoN,where B -is a factor related tov the increase `per `wavelength of theincreasing wave of the tube'and vAN is an increment lof the length ofthe circuit measuredyin wavelengths. The valueof theabscissa EBCAN isnot a simple function of length, since the valuooftB, depends onattenuation,` whichin turndepends uponpN, the length of the circuit inwavelengths measured from the tube output. Where the loss is uniformthis is simply BCN and is proportional to length. VWhere it isnon-uniform, thje expression represents a lstep by step summation ofincrenlental` lengths.

various `symbols and. quantities defined above fand lthose'which :willbaden-ned later are substantially: -the same, as those deiinedin Theoryof-the Beam-.Type Traveling-Wave Tubejby I. R. Pierce, `appeaning in theProceedings of the LRLE., February 1947, volume `3'5, pageA 111,. and inTraveling-Wave Tubes, by I. R. Pierce, appearing in the` `lfelliSystem.Technical Journal, January 1950, volume 29, page l. Y

In a practical design itl is also ydesirable to avoid having too` muchgain in excess of attenuation in anysection of the tube in .order tomaintain stability `and avoid bad impedance elfects In the describedtubefor instance, it appears undesirable to all'ow' net gain to `.exceedthe passive loss. :bynmore thank twenty decibels` in; any :section ofthetube. Otherwise, instability may occur. This limitation is shown lbycurve Bt of FIG. 7, whichtgives the minimum total attenuation allowablebetween any point fand-the output. Generally, the curve ofattenuation.per unit length .should` lie below curve A and the total attenuationshould be above curve B.

Curve B `of FlG. 7 represents minimum total attent'lation ELAN plottedagainstnormalized` gain to outputEBCAN.` The various `quantities are as'previously described.

Dashed-line curves` C-of FIG. 7 are an approximation to the `attenuationAper unit length corresponding to oui-ve B for different values of" thetube parameter QC, where Q isa parameter related to thebeam and circuitcoupling, and Cis the parameter for gain per: unitA length previouslydefined. Curves C` correspond to thedashed curve of FIG. 6,andindicateiorrthe designated value of. QC, the minimum value of theloss factor .L/C'whicli is `consistent with stability. `in `apracticaltube, there.- fore, the `attenuation .per unit length shouldbesuch, that the loss factor L/Clies belowcurve .A `andabove theappropriate curve C There are a number `of ways infwhichrthe attenuationmaybe distributed consistently with the limitations which have `been,imposed. A few of these are illustrated in FIG. 8, along with severaltypes of distributionwhich have been used in the past.

In the past, all of the attenuation has often been concentrated ina veryshort section. Such an arrangement is illustratedV by the solid ycurveof EIG. 8A, which .shows L/ C, the normalized attenuation per circ-uitwavelength measured in decibels, plotted against CN, the normalizeddistance to the output circuit. The gain following the attenuationshouldbe greater than thirty-three decibels in order to achieve maximumpower capacity. If the attenuatng fsection is .not short, the gainfollowing it should be forty decibels.

Also, Vshown in FIG. 8A are curves of G, the -gain to the output circuitmeasured in decibels, and LN, the total attenuation to the outputmeasured in decibels, plotted against CN. A-ll values `are for a valueof QC equal to live-tenths.

It hasibeen found that itis usually somewhat diiiicult to maintainstability and avoid long-line impedance effects in a tube using theattenuation distribution shown in FIG. 8A. Such a distribution is not,therefore, considered to be as advantageous .for power tube design as`those within the. scope of the present invention.

In the past, the .attenuation lhas also been uniformly distributed overthe major part of the` length of the tube, leaving short sections ofessentiallylossless circuit near the output and near theinput. .Such anarrangement is demonstrated by the solid curve of FIG. 8B, where L/Cisplotted against CN. The attenuation should start at a distance fromthe input end of the circuit corresponding to CN equal to at leasttwo-tenths, and should extend to the distance from the output given by`curve A of CN are also given in FIG. 8B. QC is taken as equal totivetenths.

In the distribution of FIG. SB, the attenuating section should` be longenough to accumulate suilicient total attenuation for stability. With`a` large attenuation per unit length there would still be danger ofinstability because of the` largegain in excess of attenuation in theoutput section. With losses smallenough to avoid this diculty, a verylong lossy section, and therefore a long tube, would be required, asindicated in FIG. 8B. Except for very special applications where a verylong crcuit may be used, this `arrangement would. not `be asdesirableasattenuation distributions which are in accordancewith thepresen-t invention.

FIG. 8C represents a distribution of attenuation in accordancewit-h Iafeature of the invention, Athe solid. curve representing L/C ,plottedagainst CN and the dashed curves Irepresenting G and ,LNplotted againstCN. 'QC is `again .taken` as five-tenths. This distribution may be had,as has been previously described, by providing an essentially losslesscircuit from the-output back as, far as stability considerations permit,and then providing attenuation per unit lengthequal ,to` or greater thanthe net gain per unit length Within the attenuating region. Theattenuating region should be extended toward the input end of the tubeto a point corresponding to a level of at' least thirty-three decibelsbelow the output, under low level conditions of operation. From thispoint, extending toward the input, the loss may be heavily concentratedin a` short section until attenua-tion greater than the net gain of thetube hasbeen accumula-ted. From the end ofthe heavy loss to the inputtisa lossless circuit of length cor` responding toa CN- equal to or greaterthan two-tenths.

'Ihedistribution illustrated inE'IG. `8C allows twenty decibels of gainin the output section and provides an over-all attenuation in theopposite direction of the electron llow ten decibels greater than thetotal gain. This type of characteristic is generally the most favorable,because it provides, in most cases, the shortest tube consistent withobtaining the maximum power output and maintaining stabilityrequirements. Furthermore, since most of the attenuation isconcentrated, it detracts from the over-all gain less than if more ofthe loss were distributed.

FIG. 8D represents another attenuation distribution in accordance with afeature of the present invention. As in the other figures justdiscussed, the solid curve of FIG. 8D shows L/ C plotted against CN andthe dashed curves show G and LN plotted against CN. QC is once moretaken as equal to five-tenths. The distribution shown uses the maximum.attenuation consistent with power output in order to hold long-lineeffects and instability problems to a minimum. To determine such adistribution, it is assumed that if the level at any point in anonauniform attenuating region is below six decibels less than theoverload level in a uniform region of the same attenuation per unitlength, the output level will not be seriously affected. Using curve Aof FIG. 7 and obtaining the attenuation as la function of CN bygraphically integrating BCAN, the solid curve of FIG. 8D is obtained asthe distribution providing the maximum protection against instabilityand long-line impedance effects. The attenuation extends to a distancefrom the input of CN equal to at least two-tenths.

It will be noted that the FIG. 8 attenuation distribution curves are, ina sense, inverse to those of FIG. 5. In FIG. 8, the abscissa representsdistance measured from the output end of the circuit, while in FIG. itrepresents distance measured from the input end.

All of the curves of FIG. 8 are derived for tubes with a space chargeparameter QC of the order of five-tenths. The curves of FIG. 9correspond to those of FIG. 8 except that QC is taken for threedifferent values and the resulting iield strength distributions are notindicated. The values of QC for which the curves are derived are zero,five-tenths, and unity, respectively.

The curves in FIGS. 8 and 9 are for a minimum length of tube consistentwith the limitations imposed. The tube may be made of any length, andthus of any gain, by making the additional circuit length with coldattenuation per unit length equal to or greater than the net gain perunit length and placing the additional length adjacent to the region ofheaviest attenuation.

In general, a distribution of attenuation along the circuit of atraveling-wave tube in accordance with the fea-y ture of the inventionunder discussion enables the tube to yield high power output `and tooperate at a high degree of efficiency. Instability and long-lineimpedance effects are minimized. To recapitulate, this feature of theinvention comprises a distribution in which substanf tially losslesssections of the circuit are left at both ends of the tube and in whichthe Aattenuation is distributed along a center section intermediate theend sections so that the attenuation per unit length is at least severaltimes greater near the input or upstream end than it is near the outputor downstream end. In one embodiment of the invention the center sectionof distributed loss comprises a relatively short section of very highattenuation per unit length followed by a relatively long section ofonly moderate attenuation per unit length. In another embodiment, theattenuation per unit length in the center section is at a maximum nearthe input end of the section and decreases gradually to substantiallyzero at the output end. In both embodiments, the total attenuation overthe length of the traveling-wave circuit is comparable in magnitude tothe net gain of the tube.

In the described traveling-wave tube, the attenuation or loss materialis deposited upon'the'helixsupporting 12 rods 35.- In other varieties oftraveling-wave tubes," other4 appropriate means of physical distributionmay be ernployed. I i

In the operation of traveling-Wave tubes, the dimensions of the electronstream should generally be main-I tained over relatively long distances.Otherwise, elecf" trons tend to strike the traveling-wave circuit andbe-4 come lost or tend to drift far enough away from the circuit so thatcoupling is lost. In either event, the gainI of the tube may beseriously affected. Usually, beam dimensions have been maintained overrelatively long distances by placing the Whole beam and the associatedstructure in the strong uniform longitudinal magnetic eld produced by asolenoid. Such a solenoid is usually large and bulky enough to surroundthe whole tubethroughout its length and considerable focusing power isrequired, thereby reducing the over-all power eii'ciency lof the system.

In accordance with another feature of the present nvention, permanentmagnets are employed as auxiliaries to a solenoid in a compositesys-tem. A relatively small solenoid may be used and the bulk and thepower re-n quirements of the focusing system are considerably re# ducedfrom those of the focusing systems use d in the past.

As shown in FIG. 3, a relatively small diameter solenoid 48 fits aroundenvelope 21 between wave guides. 45 and 46. In order to extend thefocusing iield past the input and output circuits, Va pair of relativelyshort permanent bar magnets 58 bridge input waveguide 45, while asimilar pair oflbar magnets 68V bridge output wave guide 46.

The axial magnetic lield is kept uniform at the junc-f tion of thesolenoid 48V and the permanent Vmagnets 58 or 68 by overlapping themechanical components so that the end turns of the coil 48 are nearlyllushwith the inner surfaces of the end plates 52 and `53. Thus theleft-hand end of solenoid 48 is flush with the lefthand face of endplate 52, while the right-hand end of solenoid 48 is -llush with theright-hand facepof end plate 53. The solenoid tield should, it is to benoted, be substantially equal to the permanent magnet ield in strength.Y

In order to obtain field uniformity within the solenoid 48, the coil iseffectively shielded from external leakage fields of the permanentmagnets by shields 50'and'54.

FIG. l0 shows the distribution of the magnetic field at the input end ofthe described tube. For' clarity, the tube itself is not shown and thesection is takenV at righ-t angles to the section shown in FIG. 3. Theposition occupied by input wave guide 45 when the tube is insert; ed isVshown by the dashed lines.

Through the employment of the feature of the present invention underdiscussion, it is found that not only is the focusing system morecompact than those previously in use but also only about one-twentiethof the focusing power is required. Tubes making use of this feature ofthe invention are suitable for use in installations where space is at apremium. The physical size of the focusing system is small and thesolenoid power supply need not be as large as is required if only asingle large` diameter solenoid is'employed.

As has been noted previously, a number 0f soft steel transverse platesor discs 60, 61, 62, and 64 are employed to straighten out possibledefects in the magnetic focus.` ing field. The plates are carefullyaligned so that their planes are perpendicular to the direction of thedesired magnetic field, and transverse components. are therebyeffectively removed.

The transverse plates may .have any convenient shape dictated by aparticular application. In order to make the field strength more uniformalong the axis of a long permanent magnetic structure, the transverseplates may be cupped or bent, as shown in `F'IGS..11A and 11B. Suchfield straighteners formY the basis of myv coi-l.

pending application Serial No. 664,015., pled Junco, 195,7, now: UnitedStates Patent 2,1942,1 41,.iss ued June 21,119.60.

Asan alternativerto the composite focusingsystem which has beendescribed, `an all permanent magnetsystem `may .be Tused, Vvemployingnumerous y'c ransyerse s teel plates .or discs along thetraVeling-wavecircuit toeliminate irregularities lField uniformity maybeobtained bymagnetic shouting of high field regions, by controlling themagnetizationalong the Vmagnetylength,ror by usingmagnets -somewhatlonger than the electron stream which is to be focused.

B y Way `of example, four long permanent bar magnets maybe used. .Suchmagnets extend parallel to` .theftube axis and are .equallyspaced aroundits periphery. A large number of .transverse Y steel plates are `usedtol eliminate transverse irregularities in :the magneticfieldV in themanner described above. As previously stated,..in. order :to make. thefield strength more uniform along the axis .of thestructure, the`transverse plates :may be-.cupped or bent, as shown ini-FIGSNI'IA `and'1 1B. Shunting `of the longitudinal field is obtained by means of`theufiaps, designated `numerals 9-1 and Q2, respectively,vof FIGS. `11A`and `L1B, and can be varied by spacing theplates so that they `are,nearer together .at Ythe .ndsqof the tube thanat themiddle This methodof `shulitiug hasthe unique ,property that, while decreasing .the Vfieldnear the ends of the-structure, it increases the field at the middle.

\ :Another diflcultywwhgh has beenncountsrsd rainaths operation .of.traveling-werfe tubes is Athe reduc` on V.of gaindue to a velocityspread within the-.electron stream. The spacecharge of the electronstendsfto causen-lowering-of potential.alcns'the` axis oftherbsam- The,reduction in potential, in turn, tends to slow up theelectrons `in theAcenter .of the f-beam `andA cause o them to travel more slowly thanthose, on theoutside. Since maximum gain Ain -a `tra,veline-wave tube iSatleastfparftially dependent `on alrelativelycritical velocityseparation `between the electron stream .and -the traveling signalwave,the reduction of electron velocity :along the ,beam `axis. tends toreduce `the 4contribution `of the `central electrons `to tube gain. Theover-.all `gain of the tube, therefore, tends .to beV reduced. oyElectrons are, inA effect, wasted, and .thepower .efficiency of thetubeis reduced.

.vin Iaccordam;e with still another feature` ofthe .presentdnvention, aconvergingelectron gun is used `and 4it is shieldedfrom the magnetic`foczusingiield, .theelec- 4,tronlstrearn being subjected to `the,fieldabruptly at the point of minimum beam diameter. A type Iof electronflow -is thereby obtained in which the directaXial yelooity is constantacross ,the beam, .but `in which the electrons -have va-.tangentialvelocity fcomponent `proportional totheir-radial-distance from .thefbeamaxis. In addition,jbeam focusingproblems are minimized and a high.degree of electron `transmission without loss to thetravcling-Wave.circuit is secured.

FIGS. 3 ,and l0` bothshow how the shieldingand sudden application `ofthe iield is accomplished. Theporr,tion `of envelope Zlwhich `houses the.electron gun is surrounded by magnetic shields 56) and S7. The electrongun is, therefore, in a substantially fieldfreel region as .far as themagnetic focusing field is concerned. End `plate 55 serves toshield theelectrongun further and is `located with its right-hand face alignedwith the `point .of minimum beam diameter. The electron `stream isthereby subjected to the magnetic focusing 'eld abruptly. The directaxial velocity of the electron stream is constant across the stream, andthe gain-of thetube is not reduced by velocity variations within thestream.

In the operation of traveling-Wave tubes of the type described,stability problems are often greater atabout 'half `the normaloperatingfrequency thanat the operat- .ing frequency. The electronicband width ofthe traveling-wave amplifier is 'much Wider 'than Vthe'bandwidths off` the. signal inputV and output circuits .and the`associated; apparatus. At microwave froquenciem it o is `convenient.togopfrate ,the ,tube at frequencies higher :than thatlof maximumelectronic gainin order to have larger circuit .dimensions and thuseasier beam iocusing co ditions. Eorfinstance, the tube ,described has.muchgr4 at-4 er gain .at 2,000 megacycles than it has at`4,00()megacrcles.. where it is used. Stability problems tend. therefore, to begreater `at the Llower frequency.`

ln-.order to assure stable operation, it is desirable 2to provide muchgreater circuit attenuation than is required at the operating frequency(that is, -4J000fmssacys1es in the tubefrdescribed). Since such extra`attenuation,tends to. reduceythereectiyeness of the tube at operatingfrequenciespiteis desirable .to apply it to `the traveling-.ryavecircuit in such a way thatitproyides much greater attenuation atfrequencies lower than themperatirrg frequency than. it does at .theloperating frequenoy .may be accomplished by placing .the dissipativeorlossy/material in the yicinity of, but slightly removed -frorrnithepassive circuit @of .the rtube. VInthe.c .iescribed tube, thelossy-,materia-l may, bercoatedron all sides ofthe helix supportingrodsirbutfthosecontacting the helix 34. In.eiect, .the lossyrmateralwisthus made greater@ thsr 0n9ns-0flhs rods 3,5 away fromhelix 3.4thanonthose portions toward helix ,3.41.` `Since lthestirength .of thefield set u p :by a traveling wayerdecreasesmore rapidly Ywithfdistaneefrom the helixi` at higher frequencicsfthan atlower frequenf cies,.thedossymaterial on the outside of the rods lgives less `attenuation,at `the operatingfrequency than atthe lower zmaximumggain frequency,where the ,tube-.tends to be unstable.

. @Where-.the p1ate eficiencyof.a traveling-wavetubeis fanfimportantfactor, itmay be made `to be rmuch higher thanethe beam .efficiencybycol-lecting the electrons at muchzlowerrthan .the `beam voltage. Byplateeciency is .meantv .the :ratio ,of the output ,power to `the total1 dissirpationdessheater power. Thus, referring to FIG. 3, .collector 42is connected to a point onitlkpower supply -4'7..Whichfis negative`,withrespect to the point to which l:helix :34,is1connected.`Ithasrbeen found thatrthe ,electron ,beam.;may ,be collected atabout-one-,third-of ,-thebeam 'yoltageawithout degradingtheperformanceof .thetube The plate eiciencywis thereby increased `by a,factor :Gfltwo`orthree, `:breeding Q11 the "focusing conditions`.NV-Irenethe-.electron beam, is .collected at ldlsd foltage, anotherfactor tending tocontribilte` totube instability Vis thatof secondaryemissionrfrom the collector 42. 'Secondary electrons from collector 42tend .to ,return through the circuit, giving gain, in the reversedirection, thusucansing regeneration and, in somerinstances,.oscillation. Intord'er toavoid returningsecondary electrons, a` slightasymmetry -in rthe magnetic focusing field-isdnatroduced`nearzthecollector 42. A smalll piece .of mag- -netic ,material` such:as iron placed at one side` of the col- -lector .electrode `42introduces `a field disturbance sui- `cienttto.deflectthe.secondaryelectrons and prevent them from returning through ,the circuit.

In-a high current and voltage electron gun `ofthe type shown in FIG. .3,a discharge phenomenon :of momentary duration` similar to the` flash-arc.common in highpower tubes at lower frequency has often beenobscrved.`In the present instance, the discharge is to a low-current elec-Itrode,1the `anode `31, and may be prevented` by placing arresistance69`irl-series with the anode31 and the power supply1\47. Resistance 69should be equal to or larger `than the negativeimpedance of thedischarge. A `ten `thousand ohm resistor has been found quiteetlectiveand does not interfere with the normal operation of lthe tube.

In the operation of traveling-wave -ampliiiers, it is often found thatoutput power is lost because of the slowing-,up of the `electronbeamtaking place at` the output endof the tube. .As. the` electron beam.becomes` modulated `by 'interactionswiththe traveling-Wavm it imparts`energy,to

the wave, causing the wave to grow in amplitude. The transferred energycomm Ifrom the kinetic energy of the beam and the lbeam velocity tendsto be decreased. As the beam velocity is decreased, the beam drops outof synchronism with the wave and power is lost. This power loss may beavoided by operating the tube at la beam voltage higher than thesynchronous voltage. The beam is launched above lthe synchronousvelocity and is slowed down to near synchronism at the output end of thetube. At a slight sacrifice in gain maximum power output is secured.

It is to be understood that the `above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. An amplifying yspace discharge device which comprises means defininga path of travel for electrons, an electron source, means adjacent saidpath for directing a stream of electrons lfrom said source lengthwisealong saidV path in -a predetermined direction, and continuouselectromagnetic wave transmission means disposed along said path, saidtransmission means comprising successively, in the direction of electronow, a region of substantially no attenuation per unit length, a regionof distributed attenuation in which the attenuation per unit length isat least several times greater at the end of the region nearest saidsource than at the other end of the region, and another region ofsubstantially no attenuation per unit length, the length of said regionof distributed attcnuation being vat least as great as the combinedlengths of said regions of substantially no attenuation, the attenuationin said region of Idistributed attenuation being concentratedpreponderantly in the half of said' region nearest said source, and theattenuation per -unit length in said region of distributed attenuationbeing less than its maximum value over a major portion of the length ofsaid region.

2. An amplifying space discharge device which comprises means deiining apath of travel for electrons, an electron source, means adjacent saidpath for directing a stream of electrons from said source lengthwise-along said path in a predetermined direction, and continuouselectromagnetic wave transmission means disposed along said path, saidtransmission means comprising successively, in the direction of electronflow, a region of substantially no attenuation per unit length, :aregion of high attenuation per unit length, a region of attenuation perunit length at least several times less than the attenuation per unitylength of said region of high attenuation per unit length, and anotherregion of substantially no attenuation per unit length, the length ofsaid region of low .attenuation being at least several times as great asthe length of said region of high attenuation 4and the combined lengthsof said regions of high and low attenuation being at least as great asthe combined lengths of said regions of substantially no attenuation.

3. An amplifying space discharge device in accordance with yclaim 2 inwhich at least half of the attenuation of said wave transmission meansis concentrated in said region of high attenuation per unit length.

4. An amplifying space discharge device which comprises means defining apath of travel for electrons, an electron source, means adjacent saidpath for directing a stream of electrons from said source lengthwisealong said path in a predetermined direction, and continuouselectromagnetic wave transmission means disposed along 'said path, saidtransmission `means comprising successively inthe direction of electronflow, a region of substantially no attenuation per unit length,`a regionof distributed attenuation in which the attenuation per unit length1ismaximum at the end of the regionnearest said source and decreases'gradually throughout the length of the region to substantially zero atthe other end thereof, andanother region of substantially no attenuationperlunit length, the length of said region of distributed attenuationbeing at least as great as the combined lengths of said regions ofsubstantially no attenuation.

5, An amplifying space discharge device in accordance with claim 4 inwhich the attenuation in said region of distributed attenuation isconcentrated preponderantly in the upstream half of the region. f

6. An amplifying space discharge device which comprisse an elongatednon-conducting tubular envelope, an elongated helical conductorextending lengthwise of and within said envelope, an electron source,means to direct an electron stream from said source lengthwise throughsaid helical conductor in a predetermined direction, a plurality ofnon-conducting rods extending lengthwise of said envelope and spacingsaid helical conductor apart fromv the interior of said envelope, anddissipative material concentrated preponderantly on the surfacesV ofsaid rods away from said helix, said dissipative material beingdeposited to give said rods successively, in the direction of electronilow, a region of substantially no attenuation per unit length, a regionof distributed attenuation in which the attenuation per unit length isgreater at the end ofthe region nearest said source than at the otherend of the region and another region of substantially no attenuation perunit length, the length of said region of distributed attenuation beingat least as great as the combined lengths of said regions ofsubstantially no attenuation, the attenuation in said region ofdistributed attenuation being concentrated preponderantly in the half ofsaid region nearest said source, and the attenuation per unit-length insaid region of distributed attenuation being less than its maximum valueover a major portion of the length of said region.

7. An amplifying space discharge device which comprises an elongatednon-'conducting tubular envelope, an elongated wire helix extendinglengthwise ofand within said envelope, an electron source, electrodemeans to direct an electron stream from said source lengthwise throughsaid helix in a predetermined direction, a plurality of non-conductingrods extending'lengthwise of said envelope and spaced about theperiphery of said helix to space said helix apart from the interior ofsaid envelope, signal input coupling means in the form of a hollow Waveguide positioned at the end of said helix nearest said source, signaloutput coupling means in the form of a hollow Wave guide positioned atthe other end of said helix, dissipative material disposed along saidrods to divideA said helix into successively, the direction of electronow, a region of substantially no attenuation per unit length, a regionof distributed attenuation in which the attenuation per unit length isat least several times greater at the end of the region nearest saidsource than at the other end of the region, and another region ofsubstantially no attenuation per unit length, the length of said regionof distributed attenuation being at least las great as the combinedlengths of said regions of substantially no attenuation, the attenuationin said region of distributed attenuation being concentratedpreponderantly in the half of said region nearest said source, and theattenuation per unit length in said region of distributed attenuationbeing less than its maximum value over a major portion of the length ofsaid region, a solenoid surrounding said envelope and coaxially alignedtherewith extending between said input and output wave guides to focusthe electron stream, a pair of permanent magnets extending substantiallyparallel'to the direction of electron flow bridged across 'said inputwave guide on substantially opposite sides of said envelope to extendthe magnetic focusing field of said solenoid substantially uniformitypast said input Wave guide in the direction toward said source, and apair of permanent magnets extending substantially parallel to thedirection of electron flow bridged across said output wave guide onsubstantially opposite sides of said envelope to extend the magneticfocusing eld of said solenoid substantially uniformly past said outputwave guide in the direction away from said source.

8. An amplifying space discharge device which comprises an elongatedelectrical wave transmission circuit, a converging electron gunpositioned at one end of said transmission circuit to direct aninitially converging stream of electrons lengthwise of and in coupledrelationship with said transmission circuit, signal input coupling meansat the end of said transmission circuit nearest said electron gun,signal output coupling means at the other end of said transmissioncircuit, means adjacent said transmission circuit to supply alongitudinal magnetic eld extending substantially throughout the lengthof said transmission circuit to focus the electron stream, a plate ofmagnetic material between said electron gun and said input couplingmeans extending transversely of the electron stream, said electron gunbeing axially aligned with an aperture in said plate and said platebeing transversely aligned with the point of minimum cross-section ofthe electron stream, and a magnetic shield surrounding said electrongun.

9. A traveling-wave amplifier including a helical wave conductor, acathode adjacent one end of said wave conductor, a collector electrodeadjacent the other end of said wave conductor, a signal input wave guideat lone end of said conductor, a signal output `wave guide at the otherend of said conductor, electrostatic focusing means for directingelectrons emitted by said cathode toward said wave conductor, anaccelerator electrode between said cathode and said wave conductorhaving an orice for the passage of electrons into said wave conductor,magnetic focusing means for directing said electrons through said waveconductor comprising a pole piece surrounding the space between saidaccelerator electrode and said helix, and extending over andmagnetically shielding said cathode, a second pole piece `in thevicinity of said collector electrode, and a magnetic structure extendingbetween said pole pieces, said magnetic structure including a pai-r ofpermanent magnets extending substantially parallel to the direction ofelectron ow and bridged across at least one of said wave guides toextend the magnetic focusing iield substantially uniformly past thatwave guide. i

10. Microwave energy vacuum tube apparatus comprising a wave guidestructure for propagating microwave electromagnetic energy along an axisfrom one end toward the other end thereof, at a speed much slower thanthe velocity of light, signal input wave guide means at one end of saidwave guide structure, signal output wave guide means at the other end ofsaid wave guide structure, means for producing an electron streamdirected along said axis in the direction from said one end toward theother end, said stream producing means including a focusing electrodefor directing the electrons in said stream along parallel paths in aregion adjacent said one end of said wave guide structure, means forproducing a magnetic field aligned substantially parallel to said axissubstantially throughout the length of said wave guide structure, saidmagnetic field producing meansincluding an apertured pole piecesurrounding said region and a magnetic shield comprising `a tubularextension from said pole piece, surrounding said stream producing means,for directing the magnetic lines of force radially through said regionto make the boundary of said stream cross substantially all of themagnetic lines of force which are enclosed by said boundary in said waveguide, said magnetic field producing means further including a pair ofpermanent magnets extending substantially parallel to the direction ofelectron flow bridged across at least one of said input and output waveyguides to extend the magnetic focusing field uniformly past that waveguide.

11. An amplifying space discharge device which compriseselectron-emissive and electron collector electrodes spaced apart todefine a path of travel for electrons, an electro-magnetic wavetransmission line disposed along said path between saidelectron-emissive and electro-n collector electrodes, said transmissionline consisting substantially, in succession and in the direction fromsaid electron-emissive electrode toward said electron collectorelectrode, of a section of substantially no attenuation per unit length,a section of distributed attenuation in which the attenuation per unitlength is greater at the end toward said electron-emissive electrodethan at the end toward said electron collector electrode, and anothersection of substantially no attenuation per unit length, the length ofsaid section of distributed attenuation being at least as 4great as thecombined lengths of said sections of substantially no attenuation, theattenuation in said section of distributed attenuation beingconcentrated preponderantly in the half of said section nearest saidelectron emissive electrode, and the attenuation per unit length in saidsection of distributed attenuation being less than its maximum valueover a major portion of the length of said section, signal inputcoupling means at the end of said transmission line nearest saidelectron-emissive electrode, and signal output coupling means at the endof said transmission line nearest said electron co1- lector electrode.

l2. An amplifying space discharge device which compriseselectron-emissive and electron collector electrodes spaced apart todeiine a path of travel for electrons, an electro-magnetic wavetransmission line disposed along said path between saidelectron-emissive and electron collector electrodes, said transmissionline consisting substantially, in succession. and in the direction fromsaid electron-emissive electrode toward said electron collectorelectrode, of a section of substantially no attenuation per unit length,a section of high attenuation per unit length, a section of attenuationper unit length at least several times less` than the attenuation perunit length of said section of -high attenuation per unit length, andanother section of substantially no attenuation per unit length, thelengh of said section of low attenuation being at least several times asgreat as the length of said section of high attenuation, and the`combined lengths of said sections of high and low attenuation being atleast as great as the combined lengths of said sections of substantiallyno attenuation, signal input coupling means at the end ofrsaidtransmission line nearest said electron-emissive electrode, and signaloutput coupling means at the end of said transmission-line nearest saidelectron collector electrode.

13. An amplifying space discharge device in accordance with claim 12 inwhich at least half of the attenuation of said transmission line isconcentrated in said region of high attenuation per unit length.

14. An` amplifying space discharge device which cornpriseselectron-emissive and electron collector electrodes spaced apart todefine a path of travel for electrons, an electro-magnetic wavetransmission line disposed along said path between saidelectnon-emissive and electron collector electrodes, said transmissionline consisting substantially, in succession and in the direction fromsaid electron-emissive electrode toward said electron collectorelectrode,` of a section of substantially no attenuation per unitlength, a section of distributed attenuation in which the attenuationper unit length is maximum at the end toward said electron-emissiveelectrode and decreases gradually throughout the length of the sectionto substantially zero at the end toward said electron collectorelectrode, and another section of substantially no attenuation per unitlength, the length of said section of distributed attenuation being atleast as great as the combined lengths o-f said sections ofsubstantially no attenuation, signal input coupling means at the end ofsaid transmission line nearest said electron-emissive electrode, andsignal output coupling means at the end of said transmission linenearest said electron collector electrodeq 15. An amplifying spacedischarge device in accord-V ance with claim 14 in which the attenuationin said region of distributed attenuation is concentrated preponderantlyin the half of said region nearest said electronemissive electrode.

16. A traveling wave amplifier including a helical wave conductor, acathode adjacent one end of said wave conduc'tor, a collector electrodeadjacent the other end 'of said wave conductor, electrostatic focusingmeans `for directing electrons emitted by said cathode ltoward A'saidwave conductor, and an accelerator electrode -between said 'cathode `andsaid wave conductor having an orice for the passage of electrons intosaid wave conductor, magnetic focusing means for directing said'electrons through said wave conductor comprising `a pole `piecesurrounding the space 'between said accelerator electrode and saidhelix, and 'extending over and magnetically shielding said cathode, asecond pole piece in 'the Vicinity of said vcollector electrode, and amagnet 'extending `hetween said pole pieces.

17. Microwave energy vacuum tube apparatus rcomprising a wave guidestructure for propagating 'microwave electromagnetic energy along anaxis from Aone end toward the other end thereof, at a speed much slowerthan the velocity of light, means for producing an electron streamdirected along said axis in the direction Kfrom said one end toward theother end, said stream producing means including a focusing electrode`for directing the 'electrons in said stream along parallel paths 'in aregion `adjacent said one end of said wave guide structure, and meansfor producing -a magnetic iield aligned substantially 'parallel to saidaxis substantially throughout the length of said wave guide structure,said magnetic ield producing means including an apertured pole piecesurrounding said region and a magnetic shield comprising a tubularextension 'from said pole piece, surrounding said stream producingmeans, for directing the magnetic lines of force `radially through saidregion to Vmake the boundlary of said stream cross substantially all ofthemagnetic lines of lforce which are 'enclosed by said boundary in saidwave guide.

18. A traveling wave tube including a 'slow wavepropagating:structureofsnbstantially tubular form and Lhaving la`longitudinal axis, 'means including `an apertured pole piece forproducing a magnetic Veld Awhich has a component `radial to 'said axisin a region outside said structure `and adjacent 'one end thereof andVis Asubstantially uniform and 'parallel to said axis throughout thespace enclosed by said propagating structure, an -electron gun forproducing a'Ibeam--of electrons whose paths raresubstantially'paralleland rectilinear `at a point which is a predetermined distance Ifrom saidelect-ron gun, `said electron gun being spaced `from said pole pieces to`position said point substantially at the median of the region wheresaidma-gnet-ic :held @has a radially directed Lcompo: nent, wherebytheelectronsin said stream are deflected to enter said uniform fieldsubstantially without'radia-l vvelocity -but with a helical motionlabout said axis.

`19. Microwave energy vacuum tube vapparatus :comprising a wave guidestructure for propagatingrrucrowave electromagnetic energy along an axis'from .one Iend to- Wardthe other endlthereof vat a speed much slowerthan the i-velocity of light, means including a cathode Afor pro-Iducing an electron vstream directed along said axis inthe directionfrom lsaid one end toward fthe y'other end, said stream producing meansincluding fa focusing felectrode for directing 1the'elect'rons `in saidVstream -along parai-lel paths in a predetermined compact cylindricalregion-adjacent said one end of 4said wave 'guide structure, means forproducing a magnetic Yfield aligned substantially parallel to said axissubstantially throughout the length =of said wave guide structure, saidmagnetic field producing means including 1an aperltured :pole piecesurrounding Asaid region for `direct-ing the magnetic lines -of forcethrough said compact cylindrical region with radial components ofdirection therethrough, -a magnetic lshield surrounding said cathode:and focusing elect-rode and for substantially shielding the magnetic'lines of force from extension finto any region of radial componentsof-electron'velocity, `and vacuum envelope means enclosing saidfca'th'ode land said axis yand the electron-Stream spacetherealonglthroughout the length 'of said wave :guide structure.

220. A traveling-wave tube including .1a slow wvave propagatingstructure tofsubstantially tubular form Vand having allongitudinal axis,meansincluding a magnetic pole piece adjacent one end of said structureand having 2.a lportion with an `aperture substantially coaxial 'withsaid structure for producing a Vmagnetic eld which has a componentradial-to vsaid .axis in said aperture and is `substantially uniform andparallel fto said axis throughout .the space enclosed by .saidpropagating structure, Vand lmeans including `a cathode and beamvforming :electrodes for producing a .stream of electrons whose paths:become subst-antally ,parallel and rectilinear at a 'point which is apredetermined distanceifrom :said cathode, said apertured portion ofsaid pole piece being between .said cathode and beam forming electrodesVon .one side and 'said one end of saidslow wave propagating-structureon ythe other side, said last-mentioned means being -spacedfrom saidpole piece .toposition said point in saidaperture'substantially'sa't:the median of the region where said magnetic field `has la radiallydirected component.

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Alticle by Hollenberg, pages 5.2 to. 52S, Bell ASystem Technical Journalfor January `1949.

